Granular polyolefin resins produced in polymerization reactors are often compounded with conventional additives, such as ultraviolet stabilizers, antioxidants, antiblock agents, slip agents, processing aids, and other additives well-known in the art. With or without addition of such additives, granular resins are frequently pelletized for ease in handling, processing, and transportation. These processes may be carried out incorporating the use of extruders or mixers characterized by a feeding section where the granular resin is introduced to the extruder or mixer and conveyed through various processing zones in which the resin is fully (or partially) melted and mechanically mixed (typically by action of co- or counter-rotating, intermeshing or non-intermeshing twin screws, single screws, or rotors). The user might use this final form of the product for the secondary fabrication step or may require re-extrusion of the polymer to generate the final fabrication form, such as blown film, injection molded objects, etc. It is common to employ the use of a polymer melt pump (also called gear pump) to more efficiently generate the polymer pressure required to flow through the discharge section of the extrusion which typically includes, but not limited to, a screen changer (where screens are located if desired) and product-forming die plate. It is commercially undesirable to shut down the processing process to periodically change these screen devices as they foul over time so many processing lines utilize an automatic screen changer which provides for a quick exchange of screens (or screening devices) via a sliding plate arrangement. Exemplary processes are disclosed in WO 2013/137,953; U.S. Patent Application Publication No. 2013/0181364; and U.S. Pat. Nos. 5,728,335, and 8,557,154.
Undesirable characteristics of polyolefin resins which are processed in such extruders or mixers include inclusions generically termed “gels” that are apparent especially in films formed from such resins. In particular, the term “gels” refers to highly localized inclusions in the polymer, especially film made from polyolefins, which are visually distinct from the surrounding polymeric film. They are mostly due to the presence of either high concentrations of unblended polymer, polymer species different in molecular weight than the surrounding polymer matrix, unreacted catalyst and activator, product contaminants, or other types of visually distinct imperfections. The presence of gels generally lowers the value of such films, and in some cases makes the films unmarketable or not fit-for-use in the application. There have been many approaches in the past to solving this problem. One approach is to remove or reduce the size of the gels from the polyolefin material prior to forming the finished product through the use of screens/filters. (See, for example, U.S. Pat. Nos. 5,730,885, 6,485,662; and U.S. Patent Application Publication No. 2007/0100132, issued as U.S. Pat. No. 7,393,916.)
Historically, polymer processors have inserted screen mesh filters (also called screen packs) into the process stream forcing the polymer to flow through the screen(s) to filter or break up undesired species from the primary polymer stream. The screens, though, can generate significant pressure drop and are limited by what the upstream system's equipment pressure rating can tolerate (or the pressure rating of the screen holder/device itself). Many of these contained species that are desired to be filtered or dispersed into smaller species are very small in nature (typically 400 microns or less) so the screens would need to have extremely small openings to act on these species, thus generating excessive pressure drop as a result. Likewise, the screens are often very thin in profile so the secondary species can potentially elongate, stretch, or deform in shape such that they can penetrate the screen device and re-emerge on the downstream side of the screen still intact.
Others have approached the problem using gel size reduction devices. For example, U.S. Patent Application Publication No. 2013/0176810 discloses, among other things, a gel size reduction device, comprising: a gel reduction mechanism provided in a polymer flow duct in which a polymer kneaded compound flows and adapted to reduce gel present in the polymer kneaded compound, the gel reduction mechanism includes at least one or more squeezing flow paths having a flow path cross-sectional area smaller than the polymer flow duct, and a squeeze ratio S1/S2 of the squeezing flow path is set to satisfy the following relationship to generate an extensional flow in the kneaded compound flowing in the squeezing flow path: squeeze ratio S1/S2 of the squeezing flow path=25 to 180 where S1: flow path cross-sectional area of the polymer flow duct and S2: sum total of flow path cross-sectional area of the squeezing flow path. Other background references include GB 1 386 143; EP 0 816 047 A; U.S. Patent Application Publication No. 2005/035051, U.S. Patent Application Publication No. 2010/022735, U.S. Patent Application Publication No. 2013/020247; and U.S. Pat. No. 4,453,905.
Despite past endeavors, there remains a need for a solution for preparing polyolefin resins that can produce articles such as films that have no or reduced levels of gels or the ability to produce a polymer composition that includes a polymer matrix having a dispersed phase of a second polymer or resin component and/or at least one additive.